The present invention relates to a motor control system for controlling at least one electric motor, an electric thrust reverser control system including such a motor control system and a method of controlling an electric motor.
Electric motors are used in a wide and varied range of applications, many of which require the electric motors to be controlled to a varying degree. The operating parameters of the motor that may be controlled include the speed of the electric motor, the torque produced by the electric motor, and the power drawn by the electric motor. Any one or more of these parameters may be controlled and it will be appreciated that this list of parameters is not exhaustive.
One application in which such motor control is required is in electric thrust reverser control systems for aircraft jet engines. Thrust reversers are employed on aircraft jet engines during landing to ensure that the aircraft is slowed quickly. In one design, they operate by deploying cowls which deflect air from the engine, thus providing reverse thrust. It is well known to operate these cowls by the use of a hydraulic control system. Such hydraulic systems require high hydraulic power to be tapped from the hydraulic circuit of the aircraft. The use of this hydraulic fluid creates maintenance problems and the location of the hydraulic lines in the vicinity of the fan in the jet engine requires additional protection of the associated hydraulic equipment. The compactness of the engine components makes locating the hydraulic plumbing particularly difficult.
U.S. Pat. No. 6,195,601 discloses a vehicle power assisted steering system. The system seeks to control the speed of an electric motor driving a hydraulic pump by measuring the current drawn by the motor, measuring the speed of the motor, and then comparing in a look up table the target speed for the motor based on the current drawn by the motor, and then correcting the motor speed to attain the target speed.
It has been suggested in U.S. Pat. No. 5,960,626 to use electric motors to operate the thrust reverser mechanism. A synchronising mechanism is employed, as described at column 4 lines 20 to 31. However, it is also suggested that actuator synchronisation may also be xe2x80x9cimplemented by electronically controlling the motor speed of the linear actuator, and/or the displacement of the movable part of the linear actuator. The linear actuator may also have an electric motor . . . xe2x80x9d.
By using one or more electric motors to operate the thrust reverser cowls the problems associated with hydraulic control systems are avoided. In a thrust reverser system it is strongly preferred that the cowls are deployed and retracted in a synchronised fashion. Such synchronisation avoids the possibility of a jam occurring in any mechanism linking the thrust reverser cowls. To achieve this in an electrical system each motor is driven to follow a predetermined speed profile. However, under extreme conditions, such as during an aborted landing or an aborted take-off, higher loads are placed on the electric motors actuating the reverser cowls. In such extreme conditions the power required by the motor can peak to a high value, for example 30 kW, whilst ordinarily a less powerful motor, for example 15 kW, would be adequate. For aerospace applications a smaller motor offers significant weight advantages to a system.
It would therefore be advantageous to provide a motor control system that prevents the power required by an electric motor from exceeding a set value.
A possible solution is to apply a power limit at the drive stage of the motor, but this has been found to be particularly difficult to implement as motor characteristics, for example the winding resistances, vary with temperature, and therefore such a power limit would have to take these variations into account.
If during deployment or retraction, at least one of the motors reaches its power limit, it will become necessary to synchronise the motors by limiting to the slowest actuator speed.
According to a first aspect of the present invention there is provided a motor control system for controlling at least one electric motor driven mechanical actuator, said motor control system being arranged to drive at least one electric motor according to a predetermined motor speed profile which varies the motor speed as a function of position of the at least one mechanical actuator, wherein said motor control system is further arranged to modify said motor speed profile during operation of said at least one electric motor to ensure that a predetermined motor power demand is not exceeded.
Preferably the motor speed profile is modified as a function of the load acting on the at least one electric motor.
Preferably the motor speed profile comprises a motor speed versus actuator position profile. Preferably an actuator position demand signal is generated in accordance with the speed profile.
Preferably the target motor speed is a function of actuator position.
It is preferred that the motor speed profile is modified by modifying the actuator position demand signal.
Additionally, the motor control system may comprise an actuator position feedback loop arranged to generate a motor control signal in accordance with a function of a received actuator position signal and said actuator position demand signal.
Additionally or alternatively, the motor control system may further comprise actuator synchronisation means arranged to substantially synchronise the relative positions of two or more of said actuators. Preferably the actuator synchronisation means is arranged to further modify the motor speed profile.
Preferably the predetermined motor power demand is substantially equal to the maximum rated power of the at least one electric motor. Additionally or alternatively, the motor speed profile may be stored in a permanent memory store or maybe selected from a plurality of alternative motor speed profiles and stored in a temporary memory store.
Preferably an estimate of the load being supplied by the motor is derived from the torque being supplied by the motor and the rate of rotation of the motor.
The present invention utilises the knowledge that (at least in an ideal motor) torque is proportional to the current in the motor windings. Thus the need to include mechanical torque sensing devices is avoided as the data necessary to implement power regulation can be derived by simple measurement of motor current and motor speed.
According to a second aspect of the present invention there is provided a method of controlling an electric motor comprising supplying drive signals to said electric motor in accordance with a predetermined motor speed profile, monitoring the power demand of said electric motor, and modifying said motor speed profile to ensure that a predetermined motor power demand is not exceeded.
Preferably the motor speed profile comprises a motor speed versus actuator position profile, the actuator being driven by said electric motor. Additionally, the method may further comprise generating an actuator position demand signal in accordance with said motor speed profile. Additionally, the motor speed profile may be modified by modifying said actuator position demand signal.
According to a third aspect of the present invention there is provided an electric thrust reverser control system for controlling the actuation of at least one thrust reverser cowl comprising at least one electric motor for actuating said thrust reverser cowl and a motor control system according to the first aspect of the present invention.
Preferably at least two thrust reverser cowls are provided, each actuated by at least one electric motor. Additionally, the two thrust reverser cowls may be synchronised with regards to their relative positions. The synchronisation may be achieved by monitoring the relative motion of the thrust reverser cowls during deployment or stowage, and, if one moves ahead of another, the amount of power to that reverser may be reduced so that it slows down, thereby allowing the or each other thrust reverser to obtain the same position.